1. Field of the Invention
The present invention relates, in general, to syntactic foam composites and, more paricularly, to fiber-reinforced thermosetting resin based syntactic foam composites exhibiting a low specific gravity and a low coefficient of thermal expansion.
2. Description of the Prior Art
A continuing objective in the development of satellites is to optimize satellite payload weight. One means of achieving this objective is to reduce the intrinsic weight of various operational elements within the spacecraft. It has been recognized by the art that the desired weight reduction could be realized by replacing conventional materials, such as aluminum, with lower density synthetic composites possessing requisite mechanical, thermal and chemical stability. Included in these low density synthetic composites is a group of materials referred to in the art as syntactic foams.
Syntactic foams are produced by dispersing microscopic rigid, hollow or solid particles in a liquid or semi-liquid thermosetting resin and then hardening the system by curing. The particles are generally spheres or microballoons of carbon, polystrene, phenolic resin, urea-formaldehyde resin, glass, or silica, ranging from 20 to 200 micrometers in diameter. Commercial microspheres have specific gravities ranging from 0.033 to 0.33 for hollow spheres and up to 2.3 for solid glass spheres. The liquid resins used are the usual resins used in molding reinforced articles, e.g., epoxy resin, polyesters, and urea-formaldehyde resins.
In order to form such foams, the resin containing a curing agent therefor, and microspheres may be mixed to form a paste which is then cast into the desired shape and cured to form the foam. The latter method, as well as other known methods for forming syntactic foams, is described by Puterman et al in the publication entitled "Syntactic Foams I. Preparation, Structure, and Properties," in the Journal of Cellular Plastics, July/August 1980, pages 223-229. When fabricated in large-block form, such foams possess a compressive strength which has made them suitable for use in submerged structures. In addition, the more pliable versions of the foam are utilized as filler materials which, after hardening, function as a machinable, local-densification substance in applications such as automobile repair and the filling of structural honeycombs. Despite these characteristics of adequate compressive strength, good machineability, and light weight, such foams lack the degree of dimensional and thermal stability required to render them applicable for the spacecraft environment. More specifically, syntactic foam systems tend to exhibit varying filler orientation and distributions within the geometrical areas in a molded intricate structure, which limits the structural intricacy that can be achieved, as well as reducing dimensional stability. If syntactic foam systems are too highly filled, sacrifices are made in moldability, coefficient of thermal expansion, strength, density, dimensional stability and stiffness. Moreover, such foams tend to exhibit poor adhesion to metallic plating which is required to form the desired product, such as an antenna component.
In order for the syntactic foam to be useful as a substitute for aluminum in antenna and antenna microwave components in a spacecraft, the foam must have the following characteristics.
(1) The material must have a specific gravity of 1.00 or less, as compared to a specific gravity of 2.7 for aluminum. PA1 (2) The material must have a linear coefficient of thermal expansion (.alpha. or CTE) comparable to that of aluminum, preferably close to 13.times.10.sup.-6 in/in/.degree.F. (23.times.10.sup.-6 cm/cm/.degree.C.) or less. Thermal distortion of antenna components subjected to thermal cycling in the extremes of the space environment is a major contributing factor to gain loss, pointing errors, and phase shifts. PA1 (3) The material must meet the National Aeronautics and Space Administration (NASA) outgassing requirements to insure that the material does not release gaseous component substances which undesirably accumulate on other spacecraft parts in the outer-space vacuum. PA1 (4) The material must have long-term stability, as required for parts exposed to the space temperature environment (e.g., -100.degree. F. to 250.degree. F. or -73.degree. C. to 121.degree. C.) for extended periods of time, such as 10 years. PA1 (5) The material must be capable of being cast into complex configurations in order to form component parts for antenna structures, such as waveguides or antenna feed distribution networks.
The art, until the present invention, has been unable to satisfy these requirements and particularly the requirement for a low coefficient of thermal expansion (.alpha.). Thus, known epoxy resin based syntactic foams filled with 10 to 30% by volume hollow microspheres generally have a .alpha. in the range of 17 to 36.times.10.sup.-6 in/in/.degree.F. (30 to 65.times.10.sup.-6 cm/cm/.degree.C.).
A need, unsatisfied by existing technology, has thus developed for a syntactic foam material which is both lightweight and of sufficient mechanical, thermal and chemical stability to enable it to be substituted for aluminum in physically demanding satellite environments.